This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-158729, filed May 29, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a mode converter of millimeter waves, and a gyrotron tube provided with the mode converter for the millimeter waves.
As a method of heating plasma in nuclear fusion or the like, there is known a method of irradiating the plasma with electromagnetic waves of the millimeter waves band. As an oscillation source for oscillating the electromagnetic waves of the millimeter waves band, a gyrotron tube for oscillating the electromagnetic waves in a higher mode is known, and use of such gyrotrons in heating of the plasma can be promised.
Since the higher-mode electromagnetic waves oscillated by a cavity resonator in the gyrotron has a large transmission loss in a waveguide, the waves are regarded as unsuitable for lager power transmission via a transmission channel extending over several tens of meters to a nuclear fusion furnace from the oscillation source. Therefore, a method of converting the higher-mode millimeter waves to beam-like millimeter waves able to be propagated in a free space by a mode converter, and transmitting the waves in a quasi-optical manner has been studied.
As the mode converter for converting the higher mode millimeter waves to the beam-like waves, for example, in PCT National Publication No. 504187/1991 (International Publication No. WO90/07800), a converter having a waveguide is disclosed. The waveguide has a structure in which converging mirrors and diverging mirror are alternately disposed in helical shapes inside the circular waveguide. In the converter, the millimeter waves are scattered by diverging mirror portions and gradually converged by converging mirror portions, and finally the beam-like millimeter waves are projected from the last converging mirror. In the following description, this proposal will be referred to as prior art 1.
In the waveguide having concave/convex portions, the converging mirror corresponds to a region in which an inner surface shape of the waveguide is concave in an axial direction and convex in an azimutual or azimuthal direction. The diverging mirror corresponds to a region in which the inner surface shape of the waveguide is concave in both the axial direction and the azimutual direction.
The larger an energy density of the millimeter waves in the edge of an aperture for passing the millimeter waves are, the larger diffraction and scattering effect of the millimeter waves become.
In this respect, a distribution shape of the millimeter waves in the prior art 1 are closer to that of Gaussian beam, as compared with a conventional VLASOV type quasi-optical mode converter (e.g., Vlasov, S. N. et al., 1975, Radio Engineering and Electronic Physics, vol. 21, No. 10, pp. 14 to 17, or Wada, Hashimoto, Nakajima, Electronic Information Communication Society Technical Research Report Vol. 88, No. 67, MW88-8). Therefore, the millimeter waves power density in the edge of the aperture is lowered, and the diffraction and scattering effects of the millimeter waves can be expected to decrease.
However, in the prior art 1, there is no detailed description concerning the shape of the aperture. Additionally, it is supposed that the portion shape largely influences the diffraction and scattering effects of the millimeter waves. Moreover, the shape of the aperture is described in Jpn. Pat. Appln. KOKAI Publication No. 283015/1993.
Furthermore, another mode converter is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 254802/1995. This mode converter is constituted by improving the prior art 1, reducing torsions of output millimeter wave shape and simplifying the structure, so that high manufacturing precision and low manufacturing cost can be realized. In the following description, the mode converter will be referred to as prior art 2.
For the mode converter disclosed in the prior art 1 or the prior art 2, in order to sufficiently shape the output waves and enhance a mode conversion efficiency, the converter needs to be long in the axial direction. It is described in the prior art 1 that a length of 3L0 or more is necessary in the axial direction. However, depending upon an input mode, a length of 10L0 or more is sometimes necessary in the axial direction in order to maximize the mode conversion efficiency. Here, L0 denotes 2xc3x97axc3x97sin W/tan B.
When the mode converter is incorporated into a gyrotron tube, an electron beam used in oscillation by the cavity resonator is guided by a magnetic field, gradually expanded and passed through the mode converter. The length of the mode converter must be limited that the mode converter does not contact the electron beam. Because of this limitation, the length of the mode converter sometimes has to be shortened even by deteriorating the mode conversion efficiency.
On the other hand, the mode converter in the prior art 1 or 2 has a structure in which deformation gradually increases from the circular waveguide in an inlet. In the vicinity of the inlet with a distance of about 30 mm from an inlet side, the structure is similar to that of a linear circular waveguide. When the mode converter is incorporated inside the gyrotron tube, a straight circular waveguide portion serves as the cavity resonator and possibly causes a parasitic oscillation.
An object of the present invention is to provide a mode converter with a high mode conversion efficiency, which has substantially no circular waveguide portion serving as an undesirable cavity resonator to minimize a parasitic oscillation, and a gyrotron tube provided with the mode converter.
According to the present invention, there is provided a mode converter, comprising a substantially circular waveguide having a transverse inner surface formed such that a deformation degree to a non-true circular from a true circular or a circular close to the true circular is gradually increased toward a radiation aperture side from an inlet side of higher-mode millimeter waves, for converting the higher-mode millimeter waves propagated in the substantially circular waveguide to the millimeter wave beams propagated in the space free, wherein a position starting the non-true circular transverse inner surface shape of the substantially circular waveguide is in a range of 0 to 5 mm toward the radiation aperture from the inlet side.
Moreover, according to the present invention, there is provided a gyrotron tube provided with the mode converter having the aforementioned structure.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.